1. Field of the Invention
The present invention relates generally to an after burning preventive system for an internal combustion engine and particularly to a system for feeding additional air into an internal combustion engine of a motor vehicle during deceleration thereof to prevent the engine from being fed with an excessively enriched air-fuel mixture which causes afterburning thereof in the exhaust system of the engine.
2. Description of the Prior Art
As is well known in the art, with an internal combustion engine an air-fuel mixture drawn into the engine is temporarily excessively enriched and afterburning of unburned components in exhaust gases of the engine takes place in the exhaust system when the throttle valve of the intake system such as the carburetor is closed during a gear change operation of the transmission of a motor vehicle having the engine or deceleration of the vehicle. As a solution to this problem, the engine is provided with an after burning preventive system which includes an additional air supply passageway providing communication between the atmosphere and the intake passageway downstream of the throttle valve, a flow control valve for controlling the flow of air passing through the additional air supply passageway, and a diaphragm unit for operating the flow control valve. The diaphragm unit has a vacuum chamber receiving the engine suction vacuum and an accumulator chamber separated from the vacuum chamber by a flexible diaphragm and having a predetermined volume. The diaphragm is operatively connected to the flow control valve. When the vacuum differential between the vacuum and accumulator chambers is increased above a predetermined value, the flow control valve is operated by the diaphragm to open the additional air supply passageway. Additional air is temporarily fed into the intake passageway through the opened additional air supply passageway thereby to prevent an air-fuel mixture drawn into the engine from being excessively enriched and to prevent afterburning of engine exhaust gases in the exhaust system.
However, with a conventional after burning preventive system the accumulator chamber has communicated with only the vacuum chamber through an orifice formed through the diaphragm. As a result, the flow control valve has been opened only while the vacuum differential between the accumulator and vacuum chambers is above a predetermined value. Since the opening time of the flow control valve has been relatively short and at a value within the range of about 1.25 to 1.65 seconds, the conventional after burning preventive system has been capable of maintaining the open condition of the flow control valve in the case that the vehicle is temporarily decelerated. However, in the case that the vehicle is continuously decelerated as when the engine is employed as a brake of the vehicle which descends a lengthy slope or in the case that the vehicle is decelerated at a steady low speed with the opening degree of the throttle valve at a small value near an opening degree for idling of the engine, it has been impossible to maintain the open condition of the flow control valve and therefore to prevent afterburning of engine exhaust gases in the exhaust system.